Tc 9-524 fundamentals of machine tools

Counterboring
When counterboring, mount the tool into the drill chuck and
set the depth stop ‘mechanism for the required depth of
shoulder cut. Set the speed to approximately one-half that for
the same size of twist drill. Compute for the actual cutter size

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and not the shank size when figuring speed. Mount the
workpiece firmly to the table or vise. Align the workpiece on
the center axis of the counterbore by fitting the pilot into the
drilled hole. The pilot should fit with a sliding motion inside
the hole. If the pilot fits too tightly, then the pilot could be
broken off when attempting to counterbore. If the pilot fits too
loosely, the tool could wander inside the hole, causing chatter
marks and making the hole out of round.
Feeds for counterboring are generally 0.002 to 0.005 inch
per revolution, but the condition of the tool and the type of
metal will affect the cutting operation. Slow the speed and
feed if needed. The pilot must be lubricated with lubricating
oil during counterboring to prevent the pilot seizing into the
work. Use an appropriate cutting fluid if the material being cut
requires it. Use hand feed to start and accomplish
counterboring operations. Power feed counterboring is used
mainly for production shops.
Spot Facing
Spot facing is basically the same as counterboring, using the
same tool, speed, feed, and lubricant. The operation of spot
facing is slightly different in that the spot facing is usually
done above a surface or on a curved surface. Rough surfaces,
castings, and curved surfaces are not at right angles the cutting
tool causing great strain on the pilot and counterbore which
can lead to broken tools. Care must be taken when starting the
spot facing cut to avoid too much feed. If the tool grabs the
workpiece because of too much feed, the cutter may break or
the workpiece may be damaged. Ensure
securely mounted and that all backlash
drilling machine spindle.
TAPPING
that the work is
is removed from
Tapping is cutting a thread in a drilled hole. Tapping is
accomplished on the drilling machine by selecting and drilling
the tap drill size (see Table 4-5 in Appendix A), then using the
drilling machine chuck to hold and align the tap while it is
turned by hand. The drilling machine is not a tapping
machine, so it should not be used to power tap. To avoid
breaking taps, ensure the tap aligns with the center axis of the
hole, keep tap flutes clean to avoid jamming, and clean chips
out of the bottom of the hole before attempting to tap.